The invention relates to packaged integrated circuits (ICs). More particularly, the invention relates to a method for marking packaged ICs.
ICs are normally xe2x80x9cpackagedxe2x80x9d prior to sale, e.g., mounted in plastic or ceramic carriers that protect the IC. The IC package is then typically mounted on a circuit board, while xe2x80x9cpackage pinsxe2x80x9d incorporated in the package allow electrical contact (through fragile internal xe2x80x9cbond wiresxe2x80x9d) between the pads of the IC and traces on the board. Commonly used packages include dual in-line (DIP) packages, ball grid arrays (BGA), and many other well-known packages.
Because many different ICs are generally available in a given package, packaged ICs are normally marked with both product tracking information and product identity information prior to sale. The product tracking information typically includes the wafer fabrication lot number and the date on which the wafer was fabricated, as well as the assembly lot number and the date on which the packaged IC was assembled. This information allows the manufacturer to track the manufacturing process and assists in solving manufacturing problems. Product identity information typically includes designations for the product type, package type, and performance (speed grade) of the IC.
FIG. 1 shows a common package marking flow. Typically, an IC device is tested briefly while still on the wafer (step 100). ICs failing the tests are discarded (step 102). ICs passing the tests are packaged (step 104), then marked on the back with product tracking information (step 106). After this step, the packaged IC looks as shown in FIGS. 1A (bottom view) and 1B (top view).
After the ink on the bottom of the package has cured, the package is marked on the front (step 108) with additional information intended for the customer, typically product identity information. Product tracking information may also be included. For example, as in FIG. 1C, a marked IC may display the company name and logo, an alphanumeric string including the product identity information (PRODUCTIDENTITY), and one or more strings including the product tracking information (PRODUCTTRACKING). FIG. 1D shows the marked information in more detail. The IC of FIG. 1D displays the product identity information in the form of a product designator (Prod), package identifier (pkg), and speed grade (spd-grade). Product tracking information includes the wafer lot, processing date, assembly lot, and assembly date.
Because the actual performance of the device is not known at this time, the highest specified performance is typically included on the front markings. Alternatively, the speed grade can be omitted from the markings, and added later after the performance has been determined.
The marked packaged IC is then subjected to further tests (step 110) to determine full functionality and performance. ICs failing the tests are discarded (step 112). If a speed grade was marked on the IC in step 108, then the ICs are tested against the marked speed grade. Otherwise functional ICs in any lower speed grade will fail the tests and must then be discarded. If no speed grade was marked on the IC in step 108, the actual, measured performance can be marked on the IC in a third marking step (not shown). Because marking is a slow and expensive process, it is common for IC manufacturers to simply mark the fastest speed grade and accept the losses of discarding slower ICs, for at least a portion of their output.
The fully tested and marked ICs are then stored until a customer order is received (step 114), at which time an IC is selected and shipped to the customer (step 116).
This straightforward process has several deficiencies. If each package is marked with the highest specified speed grade in step 108, some slower devices must be unnecessarily discarded. If the speed grade is not marked until later in the process, a total of three marking steps must be performed. Even in this case, the fabrication, testing, and marking process may result in a distribution of ICs that does not correspond to the distribution desired by customers. For example, a fast IC is typically more expensive than a corresponding slow IC. Therefore, customers who do not need fast ICs are not willing to pay a premium for them, yet the fabrication, testing, and marking process may result in a large number of ICs marked xe2x80x9cfastxe2x80x9d and a small number of ICs marked xe2x80x9cslowxe2x80x9d. In this case, it is common industry practice to sell a fast IC at the price of a slow IC, and in these circumstances it is desirable to mark the IC package with the speed grade for which the customer paid, not the speed grade at which the packaged IC functioned when tested. Further, customers sometimes wish to purchase unusual combinations of, for example, speed grades and temperature ranges, or even to purchase speed grades or temperature ranges not normally tested for. In this case, a manufacturer can easily test the product to the customer""s specifications. However, the product has already been marked in a fashion that may not be consistent with the customer""s needs. Therefore, it is not uncommon for a manufacturer to deliberately remove product identity markings and replace them with new product identity markings.
The marking removal process typically involves sanding, sandblasting, a covering of paint, or laser ablation, and can for some inexpensive ICs cost more than the IC itself. Further, the physical handling of the package necessary to remove the markings can cause damage to the fragile bond wires within the package. In fact, in the case of thin packages, the potential damage is such that some of these processes cannot be used.
In addition, sanding and sandblasting mar the finish of the package, which may be unacceptable to the customer. Painting the package is sometimes messy, and often does not fully hide the previous mark due to a textural difference between the bare package and the raised surface of the marking. This overcoat can also fail over time to adhere to the package, or can be found cosmetically unacceptable to the customer. Laser removal also mars the finish of the package and may be cosmetically unacceptable.
In order to avoid the potentially expensive and damaging step of removing product identity markings, one known marking method delays adding the product identity markings until after determining the customer""s needs. This process is illustrated in FIG. 2. The product tracking information is marked on the back of the packaged IC as in FIG. 1 (step 106). The product identity information is not marked on the package at this time. The packaged IC is then tested (step 208) to determine the actual tested performance of the IC. Any IC not meeting a minimum specified performance, or not passing the functional tests, is discarded. The IC is then xe2x80x9cbinnedxe2x80x9d(i.e., stored, typically in a marked bin) according to the actual performance of the IC (step 212). The partially marked packaged IC is held until a customer order is received (step 214) and a suitable IC is selected (step 216). A suitable IC may be an IC either meeting or exceeding the customer""s performance requirements. In some cases, especially where the customer has non-standard requirements, additional testing is performed at this point in the process to verify the suitability of the selected device. The product identity information appropriate to the sale is then marked on the top side of the selected packaged IC (step 218) and the IC is delivered to the customer (step 220).
In a variation on the method of FIG. 2, after step 106, all product identity information except for the speed grade is marked on the front of the package. The packaged IC is the tested as in step 208 of FIG. 2, and the passing ICs are binned as in step 212. The partially marked packaged IC is held until a customer order is received (as in step 214) and a suitable IC is selected (as in step 216). The speed grade appropriate to the sale is then added to the markings on the top side of the selected packaged IC (similar to step 218) and the IC is delivered to the customer (step 220).
These methods are less expensive than removing and replacing the product identity markings. However, the methods of FIG. 2 still require two or three marking steps. The more marking steps, the slower the packaging process, as marking and curing the ink takes more time than most packaging steps. Further, the increased physical contact increases the likelihood of physical damage to the IC. Additionally, between receiving an order and shipping the IC the IC must be marked with ink and the ink must be cured, preferably followed by testing in case of damage during the marking process. Thus, these methods consume additional time and slow down the delivery process. Therefore, it is desirable to provide a method for marking packaged ICs that avoids the expense of removing applied markings, minimizes the number of marking steps, and avoids the necessity for inking the package after receiving a customer order.
The invention provides a method for marking a packaged IC in which only the minimum performance information is first marked on the package, regardless of the actual performance of the IC. The packaged IC is then tested to determine its actual performance. If the IC is a minimum performance device, no further marking step is required. Thus, a second marking step is avoided for all low-performance ICs. If the IC meets a performance standard above the minimum standard, then the actual speed grade is marked on the package in a second marking step. In a first such embodiment, all applicable speed grades are marked in the second marking step. The device can then be sold as any of the marked speed grades. In another such embodiment, the packaged IC is tested and marked with one or more speed grade ranges encompassing all standard performance combinations for which the IC is found to be suitable. (There may be more than one speed grade range, for example, if the IC has a potentially different speed grade at different operating temperatures.) In other words, the minimum performance and the actual measured performance are marked to form a range of performance values for which the IC is suitable. The IC can then be sold as any performance within any of the marked ranges.
The invention further provides a method for marking packaged ICs in which only one inking and curing step is required for all ICs. According to this method, all specified performances are marked on the packaged IC at the first marking. The IC is then tested to determine the actual performance, and all performance markings not applicable to the IC are removed, preferably with a laser. For example, all performances faster than the actual performance are removed. This method does require a removal step, but advantageously avoids a second inking step. Because no remarking is required after removal, and because the laser does not need to make physical contact with the IC package, this method is safer and less expensive than known methods. In a variation on this method, all performance markings remain intact, but all applicable performance markings are identified (e.g., underlined or enclosed with a laser marking). For example, all performances equal to or slower than the actual performance are identified.
A laser is much less likely to damage an IC than other known marking methods. Further, the laser process is much faster than the inking and curing process. Therefore, it is feasible to use a laser to make changes to IC package markings after receiving a customer order. Accordingly, the invention provides a method in which all specified performance combinations are marked on the packaged IC at the first marking. The IC is then tested to determine the actual performance, and the IC is binned according to its actual performance. After receiving a customer order, an appropriate IC is selected and all performance markings except those desired by the customer are removed with a laser. In another embodiment, the desired performance marking is identified (e.g., underlined or enclosed with a laser marking).